Endocrine Responses to Insulin Hypoglycaemia in the Young Calf

1. Variations in the output of glucocorticoids and catecholamines from the right adrenal gland, in response to insulin hypoglycaemia, have been investigated in calves 2-5 weeks after birth. These have been correlated with changes in the concentration of glucocorticoids and glucagon in arterial plasma. 2. Moderate hypoglycaemia for a limited period (0-1 u. insulin/kg), elicited a prompt increase in steroid output from the adrenal gland followed by a significant rise in plasma glucagon concentration. By comparison, changes in both catecholamine output and peripheral plasma glucocorticoid concentrations were found to be trivial in this group of animals. 3. Administration of a larger dose of insulin (0-5 u./kg) produced a more substantial fall in plasma glucose concentration followed by spontaneous recovery within 2-3 hr. This stimulus elicited the release of greater amounts of both cortisol and corticosterone, followed by a significant increase both in the output of adrenaline and in plasma glucagon concentration. Increase in steroid output was accompanied by an increase in adrenal blood flow and was associated with elevated concentrations of both steroids in arterial plasma. 4. The adrenal cortical response and associated changes in plasma steroid concentration were found to be transient even in response to persistent and intense hypoglycaemia (4 u. insulin/kg). The increase in plasma glucagon concentration in this group of animals was not significantly greater than that produced by smaller doses of insulin. However, substantial amounts of adrenaline (78 plus or minus 14 ng. kg-minus 1 min-minus 1; maximum; n equals 9) together with a little noradrenaline (10 plus or minus 3 ng.kg-minus 1 min-minus 1; maximum; n equals 9) were released from the right adrenal gland under these conditions. 5. Changes in adrenal blood flow could be related to adrenal glucocorticoid output in calves given 0-1 or 0-5 u. insulin/kg. In animals given the largest dose of insulin adrenal blood flow was found to increase coincidentally with rising steroid output but this hyperaemia then persisted after steroid output had subsided to values within the normal range. 6. Calves given the largest dose of insulin (4-0 u./kg) invariably collapsed and convulsed after 2-3 hr, but these symptoms could not be related to any particular endocrine response. No clinical signs of hypoglycaemia were observed in the other animals. 7. The results are discussed in relation to previous studies of adrenal function in this and other species.